The h Bar

Slereah

00:39

So anyway what happens to a geodesic trying to exit the Alcubierre bubble

Does it just converge to the horizon

DanielSank

Hi, everybody.

DanielSank

01:09

::crickets::

@AndrasDeak To be totally fair, most of the python out there in the world is python 2, so there's a pretty good argument in either case.

IMHO, it's best to use python 3 for new projects.

ACuriousMind

@Slereah I dunno, you tell me! Geodesics doing weird stuff is your area of expertise

@DanielSank ::frogs::

Slereah

Well I'm trying to construct a spacetime that has closed timelike curves but no closed timelike geodesics

My first idea was

A system of two warp bubbles

That you need to have a little trip to go from the end of the first bubble to the beginning of the second

DanielSank

@ACuriousMind ::warrior cyborg pelicans::

Slereah

But, since the warp bubble has an event horizon

ACuriousMind

@DanielSank ::humans fleeing in terror::

Slereah

01:13

I'm thinking it might be possible for a geodesic to run into the first bubble, just butt against the horizon until the end of the trip, and then run straight into the second bubble

Hence still a closed timelike geodesic

DanielSank

@ACuriousMind So... I win?

ACuriousMind

You consider the extinction of human civilization as we know it winning? You monster!

DanielSank

@ACuriousMind Dude, whatever. Haven't you ever played video games?

ACuriousMind

@DanielSank Huh?

I don't know many videogames where the eradication of mankind is a win condition, and I've played a lot of them

DanielSank

@ACuriousMind I.e. games wherin massive destruction is the point...

@ACuriousMind Oh, I see.

You're quibbling over irrelevant details :P

ACuriousMind

01:18

:P

user228700

Hey, guys :-) Does anybody mind to lend me a little help by checking if I've integrated (well, substituted the limits) correctly?

DanielSank

@Kaumudi Just ask the question.

No need to ask to ask.

user228700

DanielSank

Nitpick: your $\lambda$'s look like backward L's :)

ACuriousMind

Ooooh, those are $\lambda$s?

user228700

01:22

Yes :-P

DanielSank

Math looks right to me.

Good job.

ACuriousMind

Also, what is $\mathrm{lt}_{x\to\infty}$? Who uses that instead of $\lim_{x\to\infty}$?

user228700

@DanielSank Alright, thank you!

DanielSank

> Who uses that instead of $\lim_{x \rightarrow \infty}$?

Kaumudi, that's who.

ACuriousMind

And yes, the math looks correct

@DanielSank The implicit question is where she got that from

user228700

01:23

^ Thasright :-P I got that from my teacher.

DanielSank

Here's some fun python code:

def func():
    try:
        return 'foo'
    finally:
        <weird side effect>

@BernardMeurer

ACuriousMind

@DanielSank I'm guessing it is fun because it doesn't do the obvious thing by being the same as without the try...finally?

Unless return 'foo' can raise an exception, it should be the same

DanielSank

@ACuriousMind It's "fun" because the finally clause happens even though we've already exited the function by returning 'foo'.

@ACuriousMind Yeah, without the try/finally the code is totally different.

The weird side effect wouldn't happen.

ACuriousMind

Ahhh, Python exits functions when encountering a return. I rarely have reason to remember that

DanielSank

@ACuriousMind That's true of every language I can think of!

ACuriousMind

01:34

@DanielSank It's not true in e.g. Pascal

DanielSank

@ACuriousMind wat?

ACuriousMind

@DanielSank In Pascal, the "return" is just assigning a value to a variable that has the name of the function you are defining, and code after that assignment will happily be executed. In Delphi, the return value is defined by assigning a value to return regardless of the function's name, yet still code after this return will always be executed.

DanielSank

Oh. Today I learned!

Two languages I've never used, and never will use.

:P

ACuriousMind

I only know because for a reason I don't know Delphi was what we were taught in school

DanielSank

@ACuriousMind That's because Europe.

For some reason, European schools taught Pascal way longer than anywhere else.

ACuriousMind

01:41

My final point would be that functional languages deal with side effects in manners that probably don't allow us to speak about whether the side effect happens "before" or "after" the return.

DanielSank

@ACuriousMind You're just too clever for your own damn good, aren't you?

ACuriousMind

@DanielSank That has been suggested to me at times, yes :)

@DanielSank I recall hearing one reason is that the code is "easier" to read for beginners due to "begin...end" instead of braces.

No idea how true that is.

DanielSank

@ACuriousMind That sounds pretty false to me.

The easiest code to read, in my experience, in python.

Largely because it uses whitespace instead of braces or begin/end markers.

I'm not saying python is the best language to learn from.

I kinda think C is a reasonable first language, but I dislike some things about it enough that I hesitate to name it the best teaching language.

If C had a less stupid build system I'd like it better.

Rust would be a good option, except that I'm not sure if the ownership semantics could be ignored long enough to teach people other stuff.

ACuriousMind

Well, our teacher was also relentless about using meaningful indentation so when learning python I just had to un-train the reflex to put a semicolon at the end of a line and most things worked fine

DanielSank

Nice.

Man, a tiny little problem I have with socket I/O is snowballing into an evening of work.

Blah.

This is why I always harp on about engineering and physics being mixed: if I were a better engineer I'd be doing more physics right now instead of pfutzing with sockets.

2

vzn

02:49

@heather looks good! :) 2 suggestions. (1) in big-num the list size is equal to "s". also generally better to not have unused variables in code. but returning an unused value is common/ useful. (2) in mod2 try using range instead

86

Q: Python - Create list with numbers between 2 values?

How would I create a list with values between 2 values I put in? For example, following list is generated for values between 11 and 16: list = [11, 12, 13, 14, 15, 16]

Slereah

03:52

Is there an axiomatization of QFT that also includes like

Basic Lagrangian/Hamiltonian mechanics

They seem skittish about including those

Solenodon Paradoxus

04:08

@Slereah I hardly think so. The general formalism of Quantum Mechanics does not require Lagrangian or Hamiltonian mechanics.

Skyler

@SolenodonParadoxus what does it need then?

Solenodon Paradoxus

These are required by a procedure called canonical quantization, which is a way of building quantum theories corresponding to a given classical theory

Skyler

@SolenodonParadoxus that sounds like its skirting AdsCFT

or is actually jjust AdSCFT

Solenodon Paradoxus

@Skyler it needs:
1. A Hilbert space (or a Gelfand triple)
2. A bunch of operators acting on the Hilbert space
3. The Bourne rule

Canonical quantization uses Hamiltonian formalism to derive Quantum Mechanical theories

But it is not necessarily the only approach

@Skyler what I'm saying is - in general, the path from classical to quantum is highly ambiguous and nontrivial.

I hardly think that it can be axiomatized

Skyler

fair point

at least at our level of understanding

as a race

Solenodon Paradoxus

04:13

There's approaches like canonical quantization / geometric quantization (look it up), but they aren't supposed to work in all cases.

Skyler

since were on the topic of quantum field theories though, i actually had a quick question

Solenodon Paradoxus

go ahead

Skyler

can you transfer momentum from a particle to a virtual particle

Solenodon Paradoxus

@Skyler do you know Feynman diagrams?

Skyler

@SolenodonParadoxus superficially, I've never worked with them but can read them

i know virtual photons

can take momentum

Solenodon Paradoxus

04:15

Well virtual particles don't actually exist (thus the name :) )

See this answer of mine

Skyler

but what about something like lets say you had a proton flying through space and very close to it a virtual proton pops into existence

Slereah

@SolenodonParadoxus But the standard model does

Skyler

@SolenodonParadoxus oh, I thought they are a momentary excitation of a quantum field

Solenodon Paradoxus

Well, in terms of Feynman diagrams - virtual particles are internal lines

And yes, you can transfer momentum from the initial configuration to the final through virtual particles

Because of the momentum conservation in each node of the diagram

But I wouldn't call this "momentum transfer from real particle to virtual"

In fact, I wouldn't call it anything. The math is actually all there is to it.

Skyler

so then is something like this permitted in the math

Solenodon Paradoxus

04:18

> And yes, you can transfer momentum from the initial configuration to the final through virtual particles

It is permitted in the math

@Slereah what do you mean? Standard model is a renormalizable QFT.

Skyler

so what is something like a virtual proton, mathematically speaking

Slereah

Well

You know

It would be a Conservative Extension

Solenodon Paradoxus

@Skyler just a useful way of organize asymptotic expansions. Internal lines in the diagrams.

@Slereah I don't understand the context of your claim. Which one of my assertions are you addressing?

Slereah

Adding Lagrangian/Hamiltonian to axiomatizations of QFT

Skyler

so when we talk about vacuum energy and things like the casimir effect how does the math correspond to it

Solenodon Paradoxus

04:21

@Slereah you can't axiomatize Standard Model because, well, you can't even define Standard Model.

2

Slereah

Well yes, but that would be the point :p

Shrodinger 2016

stand·ard mod·el
nounPHYSICS
a mathematical description of the elementary particles of matter and the electromagnetic, weak, and strong forces by which they interact.

Slereah

You gotta start somewhere

Shrodinger 2016

Shrekt kid

Solenodon Paradoxus

@Slereah I don't actually see why there's gotta be a definition. Nobody seems to think of SM as fundamental, because it doesn't include gravity, has a bunch of free parameters and so on.

@Slereah I think of SM as just a very successful tool to do approximate computations in some regime of the yet-to-be-discovered final theory.

Slereah

04:23

Well you know

It would be nice to have

Solenodon Paradoxus

We have the perturbative definition.

And to definine SM rigorously you have to go beyond its domain of validity, which suggests that such a definition could be nonsensial

Skyler

what is the relationship between quantum vacuum excitations and virtual particles

Solenodon Paradoxus

@Skyler excitations of quantum vacuum are interpreted as real particles, not virtual. Virtual particles are a part of the definition of the vacuum itself. You can think of them as uncertainties.

These uncertainties, or fluctuations, are described by the usual formalism of Quantum Mechanics: the field operator and its canonical conjugate don't commute.

Also, virtual particles can be visualized as internal lines of diagrams in interacting theories

I can't seem to recall the precise relation between the two, but I'm pretty sure they relate.

Actually, what I've been trying to emphasize is: virtual particles is a faulty concept.

Skyler

noted, so are vacuum excitations generally charge neutral?

Solenodon Paradoxus

not at all

some have E/M charge

like electrons

Actually, QFT has all the answers to all your questions :)
You could try studying some introductory textbooks

Skyler

04:33

@SolenodonParadoxus any recommendations, I'm about done with all my undergraduate physics

basically I'm just wondering what happens when a charged excitation pops up in an electric field

it should experience a change in momentum, which means the source of the field also experiences one

right?

Solenodon Paradoxus

@Skyler depends on what you want to accomplish
If you want a relatively precise and comprehensive course, try Peskin&Schroeder
If you want a crash course with little technical details, try Antony Zee's book

user116211

-1

Q: Hypothetical thrust of EM Drive

I have been reading up about the EM drive which supposedly allows for reactionless propulsion. Regardless of whether or not it is real, I was trying to do a back of the envelope calculation to see how fast it could go to gauge if it would even be viable as a propulsion system if it turned out to ...

propulsion

user116211

Mainstream?

Skyler

@SolenodonParadoxus haha, Zee and all his characters in his books

Had him for a couple of courses actually

Solenodon Paradoxus

I gotta go, will be back in 2-3 hours

Skyler

04:39

later

koolman

05:15

0

Q: Altitude in tetrahedron

I was reading a book there , I saw the below question in the book . My doubt is that how they have written length of altitude is same as projection of AD on ABC . Please explain if possible with a diagram https://i.sstatic.net/jy8Ln.jpg

geometry euclidean-geometry analytic-geometry

Help me in this question

DanielSank

05:26

@Skyler hi

Skyler

@DanielSank yo, how was your thanksgiving

DanielSank

@Skyler It was aight.

Yours?

Skyler

@DanielSank well finally id say really better from being sick for like 3 weeks, i was ok enough to go see family during the weekend though and celebrate proper though

DanielSank

@Skyler nice

You should fight me.

Skyler

by that you mean smash or is there a different contest you have in mind?

DanielSank

05:34

@Skyler Gimme the Batman.

Skyler

The Ultimate "I'm Batman!" Compilation

►

Are you available some time midweek

next week

its deadweek here but i think im done with finals by next wednesday-ish

DanielSank

I'm generally pretty free.

Secret

[Immunology] scontent-syd2-1.xx.fbcdn.net/t31.0-8/…

One reason I like immunology is because of the strong reminder of RTS games

DanielSank

06:01

@EmilioPisanty I think in this comment it would be better to omit the first sentence.

koolman

06:31

Help me in above question .

I got the options a and b

But how can I get option c

@JohnRennie

John Rennie

@koolman I'm out of practice with electrodynamics I'm afraid, so I can't answer this one.

koolman

@JohnRennie ohk , if you can do it then please tell me

user116211

07:02

Hey @johnR, today is St. Andrew's Day; do you people do something in this day? I'm seeing it's sort of a big day in Scotland.

John Rennie

@MAFIA36790 St. Andrew is the patron saint of Scotland, and there'll be a lot of drunk Scots out tonight. But in England and Wales there are no special celebrations.

As it happens St. Andrew was the patron saint of the school I went to. We used to trek off to the cathedral for a (long and boring) service on St. Andrews day.

user116211

ahh; okay.

user116211

@JohnRennie wow ;)

user116211

Is there a Patron Saint for England and Wales too @JohnRennie?

John Rennie

@MAFIA36790 St. George.

user116211

07:07

okay.

John Rennie

Though quite why he's our patron saint no-one seems to know.

user116211

Let me google him first; some info would be there in Wikipedia.

user116211

> King Edward III made him the Patron Saint of England when he formed the Order of the Garter in St. George's name in 1350, and the cult of the Saint was further advanced by King Henry V, at the battle of Agincourt in northern France.

user116211

Some relevant info.

Solenodon Paradoxus

@JohnRennie how would you describe your attitude towards religion?

John Rennie

07:21

@SolenodonParadoxus disinterest

Solenodon Paradoxus

@JohnRennie I personally find the history of religion a quite interesting topic. After all, it is our past.

John Rennie

@SolenodonParadoxus it has mostly served as an excuse for homo sapiens to show what kind and gentle creatures they are.

user116211

The most interesting topic for me @Solenodon is the East-West Schism and the differences in their attitudes and practices.

Solenodon Paradoxus

@JohnRennie you can say that about quite a lot of things.
My point is - religion and beliefs are an important part of our history, and history is interesting.

user228700

@JohnRennie: Morning! :-)

John Rennie

07:25

@Kaumudi Morning :-)

Did you decide you understand mean lifetimes?

user228700

Yes. I still don't fully understand probability density functions, but I understood the derivation.

John Rennie

You don't need to use a probability density function. You can get it directly from $N = N_o e^{-t/\tau}$. But I imagine you've had your fill of mean lifetimes for a bit :-)

Solenodon Paradoxus

@Kaumudi there is a small yet fixed probability of you dying in any 1 second :)

user116211

pdfs are easy; after-all you don't need measure theory to learn them.

Solenodon Paradoxus

The probability of you dying in time $t$ is given by the exponent then

Mew

07:34

@Kaumudi ty for the video

I've solved motivation problem now btw

John Rennie

@Kaumudi see what you've started now :-)

user228700

@JohnRennie Yes :-)

Mew

I can explain after i get dinna

laerz

user228700

@SolenodonParadoxus I am very acutely aware of this :-)

user228700

@Mew Alright, I'm glad I could help. Bye :-)

user228700

07:37

I've a quick question. Why does having a large charge:size ratio make an atom less stable?

John Rennie

You need to be careful about phrases like make an atom less stable because less stable always means less stable relative to <whatever>.

user228700

Uhh.

John Rennie

A large charge and small radius means a high electrostatic potential energy, so the ion won't form unless something else in the reaction can supply that large amount of energy.

user228700

> "A large charge and small radius means a high electrostatic potential energy"

user228700

This is what I don't understand.

John Rennie

07:43

Start with a neutrl sodium atom. It doesn't take much energy to pull off a single electron to give $Na^+$.

But the remaining 10 electrons now see a higher effective nuclear charge so they shrink inwards a bit.

user228700

I guess...

John Rennie

To pull off a scond electron takes a lot more energy because the electrons are now in a deeper potential well.

And after pulling off a second electron the orbitals shrink inwards even more and the binding energy per electron gets even greater.

user228700

Why do they keep shrinking?

John Rennie

Electrons screen each other from the nucleus. In sodium the charge of the nucleus is 11+, but the outermost electrons see an effective charge of about 1+ because the inner electrons screen out most of the charge.

user228700

Yeah...

John Rennie

07:47

But when you remove electrons this screening decreases so the electrons see a greater effective charge. The result is they bind more strongly.

user228700

Yes, but we've pulled out an electron from the valence shell. Why should that make any difference to the effective nuclear charge experienced by the inner electrons?

John Rennie

In the extreme case of $Na^{10+}$ the single remaining electron sees the fullnuclear charge of 11 and is very strongly bound.

@Kaumudi the electron density is continuous for all the orbitals. They exist as diffuse clouds not precise spherical shells. So even the outermost electrons screen other electrons to some extent.

Secret

For some unknown reason, the media is currently recycling a model proposed back in October. A check had found there is actually no new paper followed after the october paper on the many interacting world model. I have no idea why the media are recycling it when it is already being mentioned a month ago

user228700

> "So even the outermost electrons screen other electrons to some extent."

user228700

That's...not an idea that I have come across before.

Secret

07:50

What is even more eyebleeding is how the media distort the conclusion of the article and make clickbait titles saying time travel is posisble

user116211

Relevant: Mean field approximation to solve Schrödinger Equation.

user116211

Central Field Theory; that's bit clear.

Secret

It is true the many interacting model is an idea I foudn interested in, but I am seriously fed up with media recycling old news

and pose them as new

user228700

I'm still unable to relate this with an increased charge...

user116211

Anyways, @Solenodon, do you know what subalgebra is? I know what algebra is though; but don't think it is relevant in the present context.

user228700

07:54

@MAFIA36790:

user228700

> "Here is a lesson in creative writing. First rule: Do not use semicolons. They are transvestite hermaphrodites representing absolutely nothing. All they do is show you've been to college."

user228700

-Kurt Vonnegut

user228700

Some others disagree:

user228700

> "With educated people, I suppose, punctuation is a matter of rule; with me it is a matter of feeling. But I must say I have a great respect for the semi-colon; it's a useful little chap."

user228700

-Abraham Lincoln

user228700

07:56

:-)

user228700

@JohnRennie ..?

user116211

@Kaumudi I like this guy; he is a Vampire Slayer.

John Rennie

I didn't understand what you mean by I'm still unable to relate this with an increased charge

Can you give an example?

user228700

@JohnRennie I meant that I still don't understand why having a large charge:size ratio makes atoms unstable .___.

John Rennie

Would $Na^{10+}$ be unstable?

user228700

08:00

Yes, I guess so.

John Rennie

If it's unstable what would it decay to?

Solenodon Paradoxus

@MAFIA36790 a subalgebra is a subspace which is algebraically closed?

user228700

But I can't tell u why.

user228700

@JohnRennie Decay? I...dunno.

John Rennie

The point is that an isolated $Na^{10+}$ ion is not unstable and will sit around quite happily forever. But the energy of the reaction $$Na^{10+} + 9e \rightarrow Na^+$$ is so high that in the presence of any other matter it pulls off electrons from the other matter.

In everyday life we only ever see $Na^+$.

So $Na^{10}+$ is only unstable relative to $Na+$ and pretty much any other atoms.

user228700

08:05

@JohnRennie Huh? By "energy", you mean energy released, yes?

John Rennie

@Kaumudi Yes

user228700

I see. Alright, I'm still a little bit lost :-| I'll think about it meself, in awhile. In the meantime, for how much longer will u be around? (I'm so sorry-I'm rushing to finish my syllabus and my brains tends to come up with quite a lot of questions ;__;)

user116211

@SolenodonParadoxus ahh; okay, thanks.

user116211

I noticed Cartan Subalgebra in the main page; I have got K-Algebra while reading about polynomials. For the sake of curiosity, is the former term dealing with polynomials? Or is it totally different @Solenodon ?

John Rennie

@Kaumudi I'm around for about four hours more.

Solenodon Paradoxus

08:11

@MAFIA36790 I never heared about K-algebra, but I suspect that it is something I know about but don't realize that it is called K-algebra

Can you provide more details?

Btw what are you reading?

DanielSank

Hi, everybody!

user116211

@SolenodonParadoxus I'm not reading anything about subalgebra or sort of that now; have noticed the term Cartan Subalgebra while going through the main Phys.SE page.

user116211

@DanielSank o/

DanielSank

@MAFIA36790 \o

:D

user116211

@SolenodonParadoxus K is a commutative ring.

Solenodon Paradoxus

08:18

@MAFIA36790 Cartan subalgebra is the maximal span of intercommuting generators of a Lie algebra.

user116211

K-algebra is an algebraic structure over $K$ with two compositions one making unitary module and other making a ring. There are other rules too.

Solenodon Paradoxus

So I suppose it is quite different from K-algebra

user228700

@JohnRennie OK. Dyou mind discussing polar coordinates again? This time, I've gots to find the electric field at a point due to continuous charge distributions.

user116211

@SolenodonParadoxus Ahh; gotcha; thanks; beyond my scope now.

user228700

Shall I ask if I come across horrible questions?

John Rennie

08:19

@Kaumudi Give me ten minutes. I'm just finishing something at work.

user228700

OK. Please carry on. I will ask when I encounter something particularly nasty (...any minute now)

user228700

I'm ready to ask! (:-P)

Mew

@SolenodonParadoxus why can't you define the standard modle?

Solenodon Paradoxus

@Mew try it

@Mew I believe they are still trying to come closer to defining quantum Yang-Mills theory.

Mew

@SolenodonParadoxus how can I, I don't know about it

@SolenodonParadoxus is the standard model just a model, or is it a theory too?

Solenodon Paradoxus

08:33

@Mew
1. Standard Model is a particular model which is based on Yang-Mills quantum field theory
2. Yang-Mills QFT is not well-defined mathematically
3. The best we can do is to write down a perturbative expansion. This allows us to do some computations perturbatively (in powers of coupling constants).
4. Perturbative computations aren't always gonna lead you to sensible answers. For example, consider a toy model of a quantum nonrelativistic particle with $k x^2$ perturbation term in the Lagrangian. A physicist would come close to the form of the propagator using perturbation theory, but he w…

5. Just like in point (4), nonperturbative Yang-Mills would lead to a bunch of interesting predictions: confinement, mass gaps, etc.

Mew

@SolenodonParadoxus what are the prerequists for learning Yang-Mills QFT

Solenodon Paradoxus

6. So we are presented with a two-fold strategy: 1) define Yang-Mills nonpeturbatively 2) do the computations. The second point seems infinitely more hard than the first, but we are still stuch in the first :)

Mew

is that normal QFT?

user228700

^ How?

Solenodon Paradoxus

08:35

@Mew if you mean perturbative Yang-Mills, then you have to understand

Free QFT, interacting QFT (Feynman diagrams & S-matrix), Lie algebras (as Yang-Mills is built on them), path integrals

Mew

ok

lots for me to learn still

is Yang-Mills the cutting edge of physics?

Is nonperturbative Yang-Mills where it's heading?

Solenodon Paradoxus

I'd say it is pretty close to the cutting edge of confirmed physics

Mew

cool

Solenodon Paradoxus

Anything beyond is just speculations

Mew

is non-pertabative Yang Mills speculation?

Solenodon Paradoxus

08:37

Including string theory, despite some people claiming otherwise :)

@Mew well, we haven't even defined the theory yet, so it is most certainly so for now :)
But it is worth mentioning that computer simulation based on lattice models (nonperturbative) confirm confinement

Mew

ok

excellent info @SolenodonParadoxus ty

Solenodon Paradoxus

You are very welcome

user228700

I might have to go take a short (=25 mins) nap in half an hour .__.

Mew

y

user228700

'Cause I usually do, at this time of day.

user228700

08:43

But if I'm in the middle of a discussion, the urge goes bye-bye...

John Rennie

I'm back. Work is done for the day and I have a fresh coffee. Life is (temporarily) good.

user228700

Awesome :-)

John Rennie

Of course life would be better if it wasn't -3C outside ...

user228700

Oh God :-/ I'm so sorry. Not to brag, but it's 29°C here (and I'm wearing socks).

John Rennie

Actually I quite like these cold clear days.

Mew

08:46

i'ts 23 here

user228700

.__. OK.

Mew

optimum

user228700

@Mew Nope. About 32°C would be optimum for my body.

John Rennie

Just the weather for discussing polar coordinates :-)

Mew

lol

user228700

08:47

Haha :-) Well, I think I figured out that bit on my own...did u see my question?

Mew

i geddit

@Kaumudi how did u solve it

user228700

I didn't. I'm unable to understand the last bit.

Mew

oh

what is K

user228700

Figured out=Understand solution. I'm just beginning to calculate this.

John Rennie

Didn't we do something like this before? Centres of gravity or something?

user228700

08:49

@Mew $1/(4 \pi \epsilon_{\circ})$

user228700

@JohnRennie Yep, center of mass.

Mew

ok ty

John Rennie

Little do you know it, but you're actually using differential geometry - specifically the Euclidean metric in polar coordinates.

user228700

Uhh .__. Euclidean metric is a term that went straight over my head. Anyhoo, that term-where did it come from?

user228700

16 mins ago, by Kaumudi

Mew

08:52

@Kaumudi why is ur book so bad at explainging stuff?

is that to make the exams harder?

user228700

Bless me. I just realized that I could've used my effing pencil to circle that term instead of taking a picture and then using a photo editor —.—

Mew

tsk tsk

user228700

@Mew I dunno, man. I've been asking myself the same question these past few months.

user228700

Any idea..? (About the term, not my screwed up textbooks)

John Rennie

The length of a line element is $Rd\theta$ so the charge is $\lambda Rd\theta$ and the field is $KQ/R^2$ = $K\lambda Rd\theta/R^2$. Yes?

user228700

08:55

::Copying...pasting::

user228700

@JohnRennie ::head-desk:: Of course. Sorry :-/

Mew

yep makes sense

I think the book should have included that explanation

John Rennie

@Kaumudi cool :-) And the $y$ component is $E\sin\theta$, which is where the $\sin\theta$ comes in.Then just integrate wrt $d\theta$.

Mew

yeah that parts easy

user228700

Yeah .__. Alright, I'll try and use me brain a little bit more before bothering u. Loads more of these to get through today.

John Rennie

08:59

@Kaumudi it's a shame you're rushing to complete everything for the exam. I don't think the basic ideas of differential geometry are that hard, and they make lots of things easier to understand. But it would be a big distraction to attempt to learn it now.

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